Fast CompactAssignment search (#8385)

* use more efficient search through candidates in offchain-election

* mark linear accessors as test-only in election-provider-multi-phase

This prevents production code which uses them from compiling.

Also write an efficient helper for getting the target index.

* doc grammar

* use faster target_index_fn in benchmarks

* unbox helper functions

* remove unnecessary import

* write lifetime after primary trait

Co-authored-by: Kian Paimani <[email protected]>

Co-authored-by: Kian Paimani <[email protected]>